Conventional dispensing pumps comprise a main body or housing, which defines a pump chamber and is held captive in the neck of a container by a collar. A piston is arranged to move telescopically within the pump chamber between a rest position and an active position. In the rest position, the piston is typically fully extended away from the pump chamber with no force acting on it other than that provided by a biasing spring. Once the pump has been primed, the rest position is typically associated with reduced pressure in the pump chamber. In the active position, a user applies force to move the piston in towards the pump chamber such that the spring is compressed and the pressure within the pump chamber is increased.
The free end of the piston (exposed outside the pump chamber) engages with a separate spout. The piston has a central dispensing passageway, which connects with the dispensing passageway through the spout. The spring provided in the pump chamber to return the piston (and hence the spout) to its rest position after dispensing is typically helical.
Finally, the pump comprises an inlet valve in the pump chamber and an outlet valve in the dispensing passageway in the piston. The inlet valve allows product to flow from the container into the pump chamber but prevents return flow from the pump chamber into the container. The outlet valve allows product to flow from the pump chamber through the spout but prevents return flow of product or air into the pump chamber.
In the simplest conventional dispensing pumps, for instance as described in U.S. Pat. No. 5,405,057, the inlet valve comprises a ball bearing, which engages in a seat around the inlet to the pump chamber from the container. When a reduced pressure is formed in the pump chamber, by the action of the piston, product is drawn into the pump chamber from the container, lifting the ball bearing off the valve seat. Further, when the piston is depressed in order to dispense the product from within the pump chamber via the spout, the ball bearing is forced back down against the inlet valve by the increased pressure created in the pump chamber. As the ball contacts the inlet valve it seals the inlet valve, so that product does not merely re-enter the container from which it first came, but rather, is forced through the outlet valve. In order for this ball bearing to operate properly it must be maintained in a position close to the inlet valve but be able to move from a sealing position over the inlet valve to an unsealing position away from the valve. This is often achieved by means of the same helical spring used to bias the piston to its rest position, acting as a cage to contain the ball bearing. This cage effect is further enhanced by the cross-section of the spring being varied such that it narrows a little way above the ball bearing to an extent that it is narrower than the diameter of the ball bearing. This thus creates a cage within which the ball bearing may have a limited amount of room to move. It should be stressed that the spring is not used to bias the ball bearing closed against the valve under typical operating conditions.
The outlet valve is provided by another ball bearing, which engages in a valve seat defined in the dispensing passageway in the piston. The ball bearing is inserted into the dispensing passageway in the piston before the spout is assembled thereto and is then retained in the piston dispensing passageway by the spout. The spout is provided with engagement means for connecting it to the piston, and is adapted to restrain the ball bearing within the piston dispensing passageway. As the product is forced out of the pump chamber, the outlet valve ball bearing lifts off its valve seat, allowing product to pass through the dispensing passageway to the spout, where it is dispensed to the user. When product is drawn into the pump chamber from the container by the reduced pressure in the pump chamber, the outlet valve ball bearing is sucked back against its valve seat, preventing air or any product remaining in the spout from being drawn back into the pump chamber. It should be stressed that there is no external force biasing the ball bearing against the outlet valve seat under typical operating conditions, other than gravity and the suction created by reduced pressure in the pump chamber which is itself created during the movement of the piston from the active position to the rest position.
Due to the inlet and outlet ball bearings not being biased against their respective valve seats a problem arises in that with the piston/spout in the rest position it is possible for product to flow from the container, through the inlet valve into the pump chamber, through the outlet valve and out of the spout, under certain conditions. These conditions could be if the dispenser is lying on its side or is upside down, or possibly under reduced pressure conditions such as in the hold of an aircraft at high altitude. Accordingly, product can leak from the dispenser thus causing inconvenience.
Attempts have been made to overcome this problem of leakage by locking the inlet ball bearing against the inlet valve. In U.S. Pat. No. 5,405,057 this is described as being achieved by being able to lock the piston in the fully active and depressed position. When in this position the spring is compressed to such an extent that the portion of the spring which has the narrower cross-section is forced downwards so that it pushes the ball bearing against the inlet valve. One way of locking the piston in this depressed position is to have mutually cooperating screw threads located on the collar and the spout. Alternatively, mutually cooperating projections, or slots and associated projections would achieve the same result.
Although the above described locking feature works well with metallic springs which regain their shape even after relatively prolonged periods of compression, it does not work well with springs that are made of plastic which suffer from so-called “creep”.